1. Field of the Invention
The present invention relates to a printer, copier, facsimile apparatus or similar electrophotographic image forming apparatus.
2. Description of the Background Art
An electrophotographic image forming apparatus usually includes a charger for uniformly charging the surface of a photoconductive drum or similar image carrier, an exposing unit for exposing the charged surface of the drum imagewise to thereby form a latent image, a developing device for developing the latent image with toner, or developer, to thereby produce a toner image, and an image transferring device for transferring the toner image to a paper sheet, OHP (OverHead Projector) sheet or similar sheet. The charger is, in many cases, implemented as a discharge type of charger.
A problem with the electrophotographic image forming apparatus is that the chargeability of toner is susceptible to the varying environment and directly effects image quality, as known in the art. Particularly, the prerequisite with a color image forming apparatus, which is extensively used today, is that the chargeability of toner be maintained stable from the color reproducibility standpoint, among others. Further, ozone, NOx (nitrogen oxides) and other discharge products derived from the discharge of the charger deteriorate the drum to thereby lower image quality and reduce the durability of the entire apparatus.
In light of the above, a current trend in the image forming art is toward positive control over the environment around the drum, which has heretofore been regarded as an error factor, for stabilizing the charging ability of toner and enhancing durability of the drum. For example, it has been proposed to configure a space around the drum as a passage structurally isolated from the other spaces and cause controlled air, e.g., room temperature, low humidity air to flow through the passage. With this configuration, however, it is difficult to replace air inside the developing device with the controlled air. While this difficulty maybe overcome if a pump, for example, delivers compressed, controlled air into the developing device, compressed air raises pressure inside the developing device to thereby cause an air stream to blow out of the developing device. Such an air stream scatters around toner when the developing device is in operation, and causes the toner to deposit on the drum, lowering image quality. In this connection, Japanese Patent Laid-Open Publication Nos. 63-159887, 5-66663 and 10-3220, for example, teach various arrangements for coping with the scattering of toner.
Various schemes have heretofore been proposed for positively controlling the environment in the developing device in order to stabilize the chargeability of toner. Japanese Patent Laid-Open Publication No. 6-19293, for example, discloses a developing device including a humidity sensor responsive to the humidity of a developer stored in the developing device. When the humidity of the developer is higher than a preselected upper limit, as determined by the sensor, a dry gas source sends nitrogen or similar dry gas into the developing device to thereby dehumidify the inside of the developing device. As the developer is agitated, the dry gas is introduced into the developer for thereby lowering the humidity of the developer.
Further, Japanese Patent Laid-Open Publication No. 7-128967 teaches a developing device configured to remove excess moisture with moisture absorbing means wrapped with a porous, moisture-permeable material. Japanese Patent Laid-Open Publication No. 2001-109263 proposes a developing device configured to remove moisture from a developer, which is collected from the developing device, by heating the developer in a depressurized condition. Japanese Patent Laid-Open Publication No. 07-072722 discloses a developing device configured to feed water to maintain the moisture content of a developer constant.
However, with any one of the conventional schemes stated above, it is difficult replace air inside the developing device, in which toner grains of short charges, for example, are floating, with controlled air for the following reason. In practice, it is impossible to introduce, in a short period of time, controlled air into the developing device without resorting to external forces while preventing toner having a mean grain size as small as several micrometers from leaking to the outside. If a pump, for example, is used to send compressed, controlled air into the developing device, then there arises the toner scattering problem stated earlier.
A problem with the developing devices of Laid-Open Publication Nos. 06-019293 and 2001-109263 mentioned earlier is that the humidity sensor increases the cost of the developing device. Particularly, in the developing device of Laid-Open Publication No. 06-019293, the dry gas fed to the developing device is apt to fling up toner and cause it to deposit on the drum, lowering image quality. The developing device taught in Laid-Open Publication No. 07-128967 has a problem that the ability of the moisture absorbing member disposed in the developing device is limited and must be replaced from time to time, resulting an increase in cost and troublesome work. The developing device taught in Laid-Open Publication No. 07-072722 is not practicable without resorting to a sophisticated structure for feeding water to the developing device.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2-253272, 5-289494, 6-19293, 6-83153, 6-202458, 9-54494, 9-81018, 10-186815, 10-213947, 11-295986 and 2002-174951.